President Obama often quotes the line from Martin Luther King Jr: “The arc of the moral universe is long, but it bends toward justice.”

It is worth considering the long arc spanning 20 years of struggle for gay rights from 1992 to 2012. The struggle has been under way for longer than 20 years, but in Vermont that span tells a story.

Twenty years ago the notion of gay marriage — or marriage equality, as the goal was later defined — was not something that gay activists contemplated. That was a year when a nasty struggle ensued in Vermont over something much more basic.

Legislators had introduced a bill that prohibited discrimination in employment, housing and other areas against gays and lesbians. One of the chief sponsors of the bill was David Wolk, who is now president of Castleton State College and then was a member of the Vermont Senate.

Opponents argued that it was not right to single out gay people for “special” legal protections. Supporters of the bill argued that specific protections were needed in the law to protect against a historic legacy of discrimination. The bill might have failed, but during the legislative session a gay man suffered a vicious and nearly fatal beating outside a bar in Burlington. Legislators gained new understanding of the need to fight against discrimination, and the bill passed.

That fall Wolk was a candidate for lieutenant governor, and as he campaigned, he faced virulent abuse, as expressed in picket signs that said: “No rights for Sodomites” and “Wolk will rot in hell.” He lost the election.

Twenty years later, in the elections last Tuesday, three states voted in referendums to legalize gay marriage. These were not decisions compelled by court decisions. They were political actions growing out of grass-roots campaigns by the people. That means that Maine, Maryland and Washington now join a list that includes Vermont, New Hampshire, Massachusetts, Connecticut, New York, Iowa and the District of Columbia granting full marriage equality. In addition, voters in Minnesota this week rejected a ballot measure that would have barred gay marriage.

What happened in those 20 years was a historic cultural shift. Vermont had a small but crucial role in 2000 when it became the first state to adopt civil unions, a term invented in the Vermont Legislature to describe what became a first step toward marriage equality. Now, gay marriage is no longer the radioactive issue it was in the past. President Obama finally felt comfortable enough politically to express support for marriage equality. To top it off, Wisconsin has elected Democrat Tammy Baldwin, who will be the first openly gay U.S. senator.

Vermont’s experience during its civil unions struggle was similar to the experience in other places as they have confronted issues of gay rights. Success for marriage equality has come when proponents put a human face on the issue. Everywhere that gay marriage has been approved, people have seen that homosexuals are not a menacing, alien other. They are our neighbors, co-workers, relatives. They are business leaders, sports stars, actors, politicians. They have families and raise children.

Opposing gay marriage are people with a range of motivation — from devout religious belief to outright hatred. As numerous states have passed measures barring gay marriage, opponents have said that the tide is turning in the direction of traditional values. But those votes preventing gay marriage have only ratified the status quo. In fact, the tide is really moving in the direction of greater tolerance and respect. A few years ago, no states guaranteed marriage quality. Now nine do.

This cultural transformation ought to provide a lesson about the nature of secular democracy. Opponents may gradually realize that, while they are free to reject gay marriage in their lives and to counsel against it wherever they can, they are not free to impose those values on society as a whole. The arena of democracy is wide, and gradually people have learned that there is no cost to society of allowing love to flourish among the diverse family of individuals that constitute humankind.

Now the arc of the universe has bent toward justice in Washington, Maryland and Maine. It is bending in the right direction.